1. Field of the Invention
The present invention relates to a Raman amplifier for Raman-amplifying signal powers of a plurality of channels included in a signal wavelength band, an operation mode switching apparatus employed in the Raman amplifier, and an optical transmission system including the Raman amplifier.
2. Related Background Art
As optical amplifiers employed in optical transmission systems for amplifying signal powers, Raman amplifiers utilizing Raman effects have been known (see Japanese Patent Application Laid-Open No. 2000-98433, for example) in addition to rare-earth-doped optical fiber amplifiers whose optical waveguide region is doped with a rare earth element (e.g., Er element). A Raman amplifier is an optical device which supplies Raman amplification pumping light to an optical fiber acting as an optical waveguide, and Raman-amplifies the power of a signal propagating through the optical waveguide. When the signal wavelength falls within the band of 1.55 xcexcm, for example, the Raman amplification pumping light has a wavelength in the vicinity of 1.45 xcexcm.
A rare-earth-doped optical fiber amplifier is formed into a module and installed in a repeater or the like in an optical transmission system. On the other hand, a Raman amplifier can not only be formed into a module and installed in a repeater or the like, but also supply high-power Raman amplification pumping light to an optical fiber line laid between repeating sections, so as to Raman-amplify the power of a signal propagating through the optical fiber line. Therefore, utilizing the Raman amplifier can lower the effective loss of signal power in the optical fiber line. Namely, the use of a Raman amplifier is favorable in terms of realizing long-distance transmission, since it can elongate the repeating section by effectively reducing the loss of signal power.
The inventor studied the prior art mentioned above and, as a result, has found the following problems. Namely, in an optical transmission system for Raman-amplifying the signal power in an optical fiber line, if the optical fiber line to which Raman amplification light is supplied breaks, then high-power pumping light will leak out from thus broken part. If the pumping light continuously leaks out of the optical fiber line as such, the pumping energy will be wasted. Also, if an optical waveguide (optical fiber) for carrying out Raman amplification breaks or other optical components fail in a module Raman amplifier, high-power pumping light will leak out, thus making it impossible to maintain a communicating state, thereby wasting the pumping energy.
For overcoming the problems mentioned above, it is an object of the present invention to provide an operation mode switching apparatus comprising a structure for making it possible to minimize the waste of energy caused by troubles occurring in optical fiber lines and the like in order to efficiently utilize the energy consumed in an optical transmission system aimed for a long-term operation; a Raman amplifier including the operation mode switching apparatus; and an optical transmission system including the Raman amplifier.
For achieving the above-mentioned object, the operation mode switching apparatus according to the present invention comprises a first detector for monitoring a change in signal power in an optical transmission line, and a control section for switching between a communication mode and a wait mode as an operation for supplying pumping light so as to adjust a power of the pumping light supplied to an optical waveguide. The first detector detects the power PS of a signal Raman-amplified in the optical waveguide to which the Raman amplification pumping light is supplied. The communication mode is an operation mode in which the power of pumping light supplied to the optical waveguide is maintained at a first level which enables Raman amplification or higher, whereas the wait mode is an operation mode in which the power of pumping light supplied to the optical waveguide is maintained at a second level, lower than the first level, including a stop of pumping light supply or less.
The Raman amplifier according to the present invention comprises an optical waveguide for Raman-amplifying a signal power when Raman amplification pumping light is supplied thereto, a pumping light source for supplying the Raman amplification pumping light to the optical waveguide, and an operation mode switching apparatus having the structure mentioned above (the operation mode switching apparatus according to the present invention). In this Raman amplifier, the power PS of a Raman-amplified signal is monitored by the first detector of the operation mode switching apparatus and, according to a result of detection from the first detector, the control section of the operation mode switching apparatus determines whether or not to switch between the operation modes for supplying the pumping light and carries out an adjustment of pumping light power including a stop of pumping light supply.
When the signal power PS changes to a set value (signal detection level) or less, the control section in the operation mode switching apparatus adjusts the power level of pumping light to the optical waveguide such that a state of the first level or higher shifts to a state of the second level or less. In this case, the switching between operation modes for supplying the pumping light can be determined easily. Also, the control section adjusts the power level of pumping light to the optical waveguide when the amount of decrease in signal power PS per unit time changes by a set value or more. In this case, a long-time fluctuation in signal power PS is kept from being erroneously determined to be a break in the optical waveguide.
The operation mode switching apparatus according to the present invention may further comprise a second detector for detecting the power PP of pumping light supplied to the optical waveguide. In this case, the control section determines the switching of operation modes for supplying the pumping light according to both of the signal power PS detected by the first detector and the pumping light power PP detected by the second detector. For example, the control section may adjust the power of pumping light supplied to the optical waveguide (switch from the communication mode to the wait mode) when the ratio (PS/PP) between the signal power PS and the pumping light power PP changes to a set value or less. Also, the control section may adjust the power of pumping light supplied to the optical waveguide (switch from the communication mode to the wait mode) when the amount of decrease in ratio (PS/PP) between the signal power PS and the pumping light power PP per unit time changes by a set value or more. In each case, a fluctuation in signal power PS caused by a long-time fluctuation in pumping light power PP is kept from being erroneously determined to be a break in the optical waveguide.
The switching between the communication mode and wait mode includes both of an operation for adjusting the pumping light power performed by the control section and an operation for connecting/blocking a pumping light propagation path itself. Namely, while an operation for adjusting the pumping light power in the control section such as the one mentioned above is carried out by directly controlling the driving current of the pumping light source, a shutter may be disposed between the pumping light source and the optical fiber line, and the control section may control the opening/closing of the shutter so as to block the propagation path of the pumping light emitted from the pumping light source if an operation for restoring the broken optical fiber line and the like can be completed in a short period of time.
The optical transmission system according to the present invention comprises an optical fiber line for transmitting signals of a plurality of channels included in a signal wavelength band, and a Raman amplifier having the structure mentioned above (the Raman amplifier according to the present invention) for Raman-amplifying signal powers of these channels. This optical transmission system is excellent in transmission quality, since the signal powers are Raman-amplified during the signal propagation. Also, the optical transmission system may be configured such that the Raman amplifier supplies the Raman amplification pumping light to the optical fiber line. In this case, the signal powers are Raman-amplified in the laid optical fiber line, whereby its repeating sections can be elongated. The optical transmission system may further comprise a bypass optical fiber line other than the main optical fiber line, and a line switch for switching the signal transmission path from the main optical fiber line to the bypass optical fiber line when the pumping light supply to the main optical fiber line is stopped or reduced (in the wait mode). In this case, the signal transmission is kept from being interrupted by an operation for restoring the main optical fiber line.